Comments in the journal Science for July 10, 2009, have questioned the results reported here, but in the same issue, the original researchers reply to the criticisms and stand by their conclusions.

An unexpected source of human DNA resets the clock on the settlement of the Americas

Archaeologist Dennis Jenkins holds the earliest direct evidence of humans living in the Americas. This 14,300-year-old piece of human feces is changing theories about how and when the continents were settled. (Courtesy Dennis Jenkins)

On a sunny, freezing-cold afternoon in late January, I steer my rented SUV off Oregon State Highway 31 and onto an unmarked dirt road. I am soon bumping through sagebrush and snow across a rutted, dry lakebed that has been empty for the past 15,000 years. After about 20 minutes I pull up in the shadow of a brown butte where I meet Dennis Jenkins, an archaeologist from the University of Oregon.

Jenkins leads me halfway up the butte over a cascade of sharp gravel, and we are soon standing in front of five dusty caves. They are nothing special to look at: just a few feet deep, their roofs barely high enough to stand under. They would have been temporary shelters at best. Which is fine, because a few minutes would have been all someone needed to deposit what is the oldest known evidence of human presence in the Americas some 14,300 years ago.

Jenkins and University of Copenhagen geneticist Eske Willerslev argue that the artifacts were made by the ancestors of modern Native Americans, then deliberately left behind. But instead of beautifully crafted stone tools like the Clovis points that have made other early North American archaeological sites famous, the artifacts from southern Oregon's Paisley Caves are pieces of crap. Literally.

Eske Willerslev, shown here in his lab in Copenhagen, used a new technique that can recover DNA from dirt and ice to find human genes in the poop from Paisley Caves. (Cotton Coulson)

This unlikely story started in 2002, 150 miles south of Bend, Oregon. Jenkins was leading a field-school excavation of Paisley Caves, a row of shallow basalt holes overlooking a prehistoric lakebed in the Great Basin--a desert plateau that covers most of Nevada and Utah, and stretches into southern Oregon, California, and Idaho. Several feet below the cave floor, Jenkins and his 26 students uncovered fragments of prehistoric life: camel and horse bones with cut marks on them, tiny fragments of thread, a handful of rocks that may or may not be tools, and more than a dozen pieces of dried-up feces. (Polite archaeologists like to call them "coprolites" in excavation reports and grant applications.)

By themselves, coprolites are nothing unusual. Excavations in desert caves turn them up regularly and museum storerooms across the American West have boxes full of carefully labeled poop sitting on shelves. So when Jenkins heard that an Oxford University graduate student was interested in experimenting on coprolites with a new DNA extraction technique that shakes genetic material loose from soil and ice samples, he was willing to give him a shot. But he was extremely skeptical anything would come of it. "I didn't know this guy from Adam," Jenkins says. "I'm open to new science, but not open to being labeled some kind of fringe scientist."

The grad student--Eske Willerslev, an enthusiastic Dane known among his colleagues for his friendly personality, foul mouth, and outstanding research--flew to Oregon in 2004 to take samples. "I was positively surprised. Some of the animal bones still had soft tissue on them, which indicated it was a really good preservation environment," Willerslev says. "And I'm not a morphologist, but some of the coprolites looked pretty human."